The selectivity of a gas sensor is a persistent challenge for most exhausted gas sensors. Currently, potentiometric oxygen sensors based on zirconia are the only reasonably successful commercial high temperature sensors which can work above 800° C. Only limited reports exist on reducing gas detection at high temperature (e.g., approximately 600 to approximately 1200° C.). Other than stability and sensitivity of a sensor at high temperature, selectivity is the most challenging issue. Researchers are trying to fabricate new materials with high selectivity; to design sensing device configurations to include a filter or physical layer; and to use new sensing technology, such as sensor arrays and impedancemetric techniques.
The selectivity of solid-state gas sensors is always challenging for all type of electrochemical sensors, especially to differentiate gases in same group (i.e. reducing gas or oxidizing gas). Researchers have devoted tremendous efforts to improve the sensing selectivity. There are several general strategies. First of all, material design is the most important and fundamental step to endow a sensor with good selectivity. Noble metal/metal oxides and coupled metal oxides are being investigated to achieve good selectivity. In addition, catalytic or physical filter layers are being employed to improve the sensor selectivity, such as Pt catalytic layer and zeolite filter layer. Furthermore, more techniques can be explored to provide more opportunities for enhanced selectivity, such as high-frequency impedancemetric technique and sensor arrays with data analysis. Selectivity becomes more challenging when comes to high temperature above 800° C.
As the most common type of harsh environment sensors, high temperature gas sensors are of paramount importance to improve combustion efficiency and control emissions. Incomplete combustion of fossil fuels, which play a dominant role as a primary energy source for automotive and power industries, leads to the emission of carbon monoxide and hydrocarbon gas. In order to reduce the pollutant emissions and to improve the combustion efficiency, high temperature gas sensors that can provide feedback in real time to combustion processes and monitor emissions are in high demand. There is a current unmet need for such sensors.
On-Board-Diagnostic (OBD) systems usually require gas sensors that can operate in harsh environments at above 500° C. and in close proximity to engines where the exhaust gases can reach temperatures close to 1000° C. To date, commercially available sensor technology for high temperature is extremely limited due to the high requirements for sensing materials and sensor performance in such harsh environments.
Recently, more attention has been given to sensing approaches, such as “impedancemetric” sensing. Impedancemetric sensing employs AC measurements at a specified frequency. This approach is related to solid-state impedance spectroscopy which is an electrochemical characterization technique that measures the cell response over a range of frequencies, typically from subhertz to megahertz. Impedancemetric techniques have been applied on both solid-electrolyte-based sensors and resistor-type sensors. Most of the known impedancemetric sensors operate at low frequency (<100 Hz) because impedance spectra of different concentrations of analyte gas overlap in the high frequency range and the sensors can only get responses at low frequency.
Optimization of a combustion process and evaluation of the exhaust gas after-treatment system are significantly important for energy efficiency improvement and toxic emission reduction, which require control and monitoring of the gas composition. (S. Akbar, P. Dutta, C. H. Lee, High-temperature ceramic gas sensors: A review, Int. J. Appl. Ceram. Technol., 3 (2006) 302-311). These systems usually require measurement of gas concentrations in the high temperature combustion environment, where solid-state electrochemical sensors are particularly suitable. Driven by tighter emission standards, besides already commercialized oxygen sensor, research of NOx sensors, CO sensors and hydrocarbon sensors is in progress. For direct on-board diagnosis (OBD) purposes, high temperature hydrocarbon sensors employed downstream of a three-way catalytic converter (TWC) can measure the limited components directly, which can provide more precise measurements than dual oxygen sensors (indirectly determining oxygen storage capacity). (R. Moos, A brief overview on automotive exhaust gas sensors based on electroceramics, Int. J. Appl. Ceram. Technol., 2 (2005) 401-413). For this kind of application, the sensor has to withstand hot exhaust gas temperatures that can reach almost 1000° C. and exhaust oxygen contents are almost zero.
It has been reported that p-n heterojunction can be used to improve sensing properties due to the depletion layer at the interface. (C. W. Na, H. S. Woo, I.D. Kim, J. H. Lee, Selective detection of NO2 and C2H5OH using a Co3O4-decorated ZnO nanowire network sensor, Chemical Communications, 47 (2011) 5148-5150; Y. J. Chen, L. Yu, D. D. Feng, M. Zhuo, M. Zhang, E. D. Zhang, Z. Xu, Q. H. Li, T. H. Wang, Superior ethanol-sensing properties based on Ni-doped SnO2 p-n heterojunction hollow spheres, Sensors and Actuators B-Chemical, 166 (2012) 61-67; H. Huang, H. Gong, C. L. Chow, J. Guo, T. J. White, M. S. Tse, O. K. Tan, Low-Temperature Growth of SnO2 Nanorod Arrays and Tunable n-p-n Sensing Response of a ZnO/SnO2 Heterojunction for Exclusive Hydrogen Sensors, Advanced Functional Materials, 21 (2011) 2680-2686). Most of these studies are focused on ZnO and SnO2, and only in low or mild temperature range (<500° C.). Composite n-p titanium oxides have been investigated at high temperature (e.g. approximately 600° C. or greater) to selectively detect CO while eliminating interference from CH4. (N. Savage, B. Chwieroth, A. Ginwalla, B. R. Patton, S. A. Akbar, P. K. Dutta, Composite n-p semiconducting titanium oxides as gas sensors, Sensors and Actuators B-Chemical, 79 (2001) 17-27).